Functions calling other functions is like normally calling functions really. Just make sure the function being called is defined above the function that is calling it. You just call it the same way you'd call it outside a function.

As you see, one_good_turn() is defined abovedeserves_another(), and both take the argument n. Then, as like you would outside of another function, one_good_turn() is called, just within the deserves_another() function, so it can be added to 2, making the result of deserves_another() equal to the result of one_good_turn() + 2.